1. Field of the Invention
The present invention relates to an intake system provided with a surge tank in an internal combustion engine.
2. Description of the Prior Art
A multicylinder internal combustion engine is usually provided with a surge tank arranged downstream of a tubular throttle body in which a throttle valve is arranged The surge tank has a volume expansion chamber and branch passageways connected to respective cylinders. The expansion chamber forms a nodal point of a pressure wave transmitted from the respective cylinders, which pressure wave is formed by alternate opening and closing of an intake valve. Therefore, the pressure wave is reflected as a nodal point from the expansion chamber to the respective cylinders, and thus an oscillatory movement of intake air takes place between the expansion chamber and the combustion chamber of the corresponding cylinder, generating a pulsating intake air movement. When the frequency of this pulsation corresponds to the open and close cycle of the intake valve, an effective intake action is attained which is called an "inertial intake" operation. Maximum efficiency is realized at a rotational speed of the engine which corresponds to the frequency of the pressure wave, resulting in a peak of torque obtained by the engine. The frequency of the intake air pressure wave is determined by the length of the intake passageway. The length of the intake passageway is usually determined so that a maximum torque characteristic is obtained at a low engine speed range.
The pressure wave generated in a cylinder at the respective intake strokes is weakened by the pressure wave generated in the intake port of another cylinder attaining subsequent intake strokes. Such an effect is called intake interference. When such an effect occurs, the increase in the output torque at the low speed range is decreased.
In order to prevent intake interference from occurring, a system is known wherein an expansion chamber is provided with a partition wall forming two sections separated from each other, the cylinders are divided into two groups between which ignition strokes take place alternately, and the divided groups are connected to the two divided sections, respectively (see Japanese Unexamined Patent Publication No. 48-34401). In this prior art, intake interference does not take place, and, therefore, an increased torque may be generated during an engine's low speed condition. However, this construction of the prior art causes the effective volume of the expansion chamber to be decreased due to the provision of the partition wall. Thus, the efficient use of air in the expansion chamber is decreased, causing a decreased torque during a high load condition of the engine.